The trunking communication system means the dedicated mobile communication system in which multiple users share one group of wireless channels and dynamically use these channels, which is a command dispatching system. The cellular trunking communication system combines the cellular communication system with the dedicated dispatching communication system, which expands the service extent of the cellular communication system, and makes the trunking communication system to be free from the limitations in function and region of the dedicated dispatch communication system. Moreover, combining the CDMA cellular communication system with the digital trunking communication system could not only improve the number of the trunking system's users in a single sector, but also could develop much more new services with the trunking system features on the basis of the high speed grouping data services.
The CDMA trunking communication system adopts the trunking system's half-duplex communication mode, i.e., in the existing CDMA cellular communication system, if the user service data sent forward within a trunking group are same, separate forward service channels are distributed to the trunking group users, so a mass of service channels might be needed to establish in the cell at the same time, the forward power raises rapidly, thereby causing the problems of forward overload, channel congestion, normal services interruption and etc, which are caused by the centralized burst of the system. At this time, the centralized burst of the system could further cause the voice activation factor (CDMA cellular communication system is the self-interference code division system, in which the transmitting power of a certain user is the interference for other users, so the call can not be kept full rate, and the non-full rate transmitting power could be reduced) of the CDMA cellular communication system invalid, supposing that the voice activation factor is 0.5, the forward capacity of the systems with trunking functions is reduced one half relative to the forward capacity of the common CDMA cellular communication system, thereby causing the whole capacity of the system reduced greatly. In this case, the solving method sharing the forward supplemental channel in the CDMA cellular trunking communication system was proposed.
Take example for CDMA2000, sharing the forward supplemental channel means that the base station assigns a forward basic channel respectively or a dedicated control channel to the sector trunking group users in the same sector (the sector trunking group user means the users belong to a same trunking group in a same sector) to deliver the signalings of the users themself. Each forward basic channel or the dedicated control channel has independent Walsh code (the channel code of variable length), same frame bias, and length code mask. The base station assigns a forward supplemental channel to the users in the same sector trunking group respectively, to deliver the users' service data. Each forward supplemental channel has the same Walsh code, frame bias, and length code mask; the service frame content are same at the same time. That is, although the data delivered by these forward supplemental channels are coded and modulated independently, but after overlapped in the final outputting end, they could be regarded as a one-way signal in practice. In the trunking communication system, the users within the trunking share the forward channel in the same cell. The trunking group users could talk with other users in the cell when calling or talking as long as press PTT (Push To Talk, button talking) button. This PIT services are widely applied to the dialogue system and the trunking system, and applied the groups of police, fire-fighting, traffic, airport aviation, building ground, large scale convention, military dispatching and the like with command dispatching functions. With the wide development of the mobile communication value-added services, CDMA trunking communication system with colony call, information sending functions has been becoming one new mobile value-added application.
As the forward supplemental channels of the trunking group users of the digital trunking communication systems are the same Walsh codes, so after the frames having same contents are modulated and frequency spreaded as well as combined, the computing manner of the trunking sectors transmitting power and the sectors transmitting power of the common CDMA systems will be different at this time. In the common CDMA systems, different users are distinguished with Walsh codes, when overlapping the uses with different Walsh codes, the power overlaps of different users are linear overlaps. The forward supplemental channels of the trunking group users are the same Walsh codes, the transmitting power of the trunking group in the sectors are obtained by linear overlap and subsequent square operation of the extent of emitting signals by each user in the trunking groups. At this time, the relationship of the forward supplemental channels transmitting powers of the users within the same trunking group of the same sector will be dependent square instead of independent overlap.
The change of the number of the trunking groups' users will cause the change of the higher and lower limits of the sector trunking group forward supplemental channel transmitting power, and the change is drastic (for example, when the number of the trunking groups' users is 20 and 1, the shared transmitting powers of the trunking group forward supplemental channels differ by 400 times). In this way, if there are too many users in the sector trunking groups, the forward supplemental channel transmitting power overlapped by the trunking group users could cause too high shared transmitting power of the forward supplemental channels even existing overload. As this power nonlinear overlap feature of the digital trunking communication systems, it is necessary to perform single power overlap control for each trunking group under each sector, causing that the shared transmitting power of the forward supplemental channels of the trunking group users of the digital trunking communication systems are controlled in the suitable range.
In summary, the nonlinear overlap of the forward supplemental channels transmitting power of the existing CDMA cellular trunking communication system with sharing forward service channels could cause the amplitude of the shared transmitting power control of the forward supplemental channels larger, and further appear the defect of the forward power overload.